In the packaging of various food products, such as cheese and meat, in a flexible wrapper, difficulty has been experienced with the development of small pinholes in the wrapper at points of stress resulting from flexing of the wrapper during shipment and handling. The pinhole development resulted in a loss of the essential barrier characteristics of the wrapper and permitted loss or gain of moisture by the package and the access of oxygen into the package with resultant spoilage of the product intended to be protected by the wrapper.
A detailed discussion of the problems in this art and one film laminate which solved these problems is set forth in U.S. Pat. No. Re. 28,554, a reissue of U.S. Pat. No. 3,445,324. The teachings of that patent as to the state of the art are incorporated herein by reference. The wrapping material of the film laminate disclosed in U.S. Pat. No. Re. 28,554 consisted of a wrapping material prepared by bonding to one surface of a cellophane sheet coated on both sides with vinylidene chloride copolymer a biaxially oriented polypropylene sheet and to the other surface a thin layer of low density polyethylene or heat-sealable polymeric material having a melting point below that of the polypropylene. Bonding in that film laminate was accomplished by gluing or polyethylene lamination.
It was determined, however, that better results might be obtained if one were able to eliminate the adhesive layer which would also eliminate the use of any organic solvents that would be released into the atmosphere. The elimination of these solvents is a very important environmental consideration and reduces the threat of any residual organic material migrating into the food product being packaged. At the same time it was recognized that it was necessary to provide a film laminate which would maintain the integrity of the product and present a grease-resistant barrier to prevent the passing through of any food oils or grease which could destroy or greatly reduce the bond between the individual layers, thereby reducing the integrity of the package formed from upper and lower layers of the film laminate of the present invention.
U.S. Pat. No. Re. 28,554 discusses in detail the formation of packages of the type contemplated for use with the present film laminate. As indicated there, the wrapper or film laminate is contemplated for use on an automatic packaging machine which forms and closes the package by means of heat-sealing. The inner layer of heat-sealable polymeric material is placed in face-to-face relationship with the inner surface of a second laminate of identical construction which permits the formation of a heat-sealed closure which is gas-tight. With products such as cheese and meat it is found advantageous to displace the air from the void space inside the package by flushing with an inert gas or by evacuation before sealing the package. The retention of this gas or vacuum and the substantial exclusion of the atmospheric oxygen is important to the proper preservation of the product.